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1. Time-course RNASeq of Camponotus floridanus forager and nurse ant brains indicate links between plasticity in the biological clock and behavioral division of labor

   https://doi.org/10.1186/s12864-021-08282-x  

   Das, Biplabendu ; de Bekker, Charissa ( January 2022 , BMC Genomics)

   Circadian clocks allow organisms to anticipate daily fluctuations in their environment by driving rhythms in physiology and behavior. Inter-organismal differences in daily rhythms, called chronotypes, exist and can shift with age. In ants, age, caste-related behavior and chronotype appear to be linked. Brood-tending nurse ants are usually younger individuals and show “around-the-clock” activity. With age or in the absence of brood, nurses transition into foraging ants that show daily rhythms in activity. Ants can adaptively shift between these behavioral castes and caste-associated chronotypes depending on social context. We investigated how changes in daily gene expression could be contributing to such behavioral plasticity inCamponotus floridanuscarpenter ants by combining time-course behavioral assays and RNA-Sequencing of forager and nurse brains.

   We found that nurse brains have three times fewer 24 h oscillating genes than foragers. However, several hundred genes that oscillated every 24 h in forager brains showed robust 8 h oscillations in nurses, including the core clock genesPeriodandShaggy. These differentially rhythmic genes consisted of several components of the circadian entrainment and output pathway, including genes said to be involved in regulating insect locomotory behavior. We also found thatVitellogenin, known to regulate division of labor in social insects, showed robust 24 h oscillations in nurse brains but not in foragers. Finally, we found significant overlap between genes differentially expressed between the two ant castes and genes that show ultradian rhythms in daily expression.

   This study provides a first look at the chronobiological differences in gene expression between forager and nurse ant brains. This endeavor allowed us to identify a putative molecular mechanism underlying plastic timekeeping: several components of the ant circadian clock and its output can seemingly oscillate at different harmonics of the circadian rhythm. We propose that such chronobiological plasticity has evolved to allow for distinct regulatory networks that underlie behavioral castes, while supporting swift caste transitions in response to colony demands. Behavioral division of labor is common among social insects. The links between chronobiological and behavioral plasticity that we found inC. floridanus, thus, likely represent a more general phenomenon that warrants further investigation.

    

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2. Connectomic analysis of the Drosophila lateral neuron clock cells reveals the synaptic basis of functional pacemaker classes

   https://doi.org/10.7554/eLife.79139  

   Shafer, Orie T ; Gutierrez, Gabrielle J ; Li, Kimberly ; Mildenhall, Amber ; Spira, Daphna ; Marty, Jonathan ; Lazar, Aurel A ; Fernandez, Maria de ( June 2022 , eLife)

   Most organisms on Earth possess an internal timekeeping system which ensures that bodily processes such as sleep, wakefulness or digestion take place at the right time. These precise daily rhythms are kept in check by a master clock in the brain. There, thousands of neurons – some of which carrying an internal ‘molecular clock’ – connect to each other through structures known as synapses. Exactly how the resulting network is organised to support circadian timekeeping remains unclear. To explore this question, Shafer, Gutierrez et al. focused on fruit flies, as recent efforts have systematically mapped every neuron and synaptic connection in the brain of this model organism. Analysing available data from the hemibrain connectome project at Janelia revealed that that the neurons with the most important timekeeping roles were in fact forming the fewest synapses within the network. In addition, neurons without internal molecular clocks mediated strong synaptic connections between those that did, suggesting that ‘clockless’ cells still play an integral role in circadian timekeeping. With this research, Shafer, Gutierrez et al. provide unexpected insights into the organisation of the master body clock. Better understanding the networks that underpin circadian rhythms will help to grasp how and why these are disrupted in obesity, depression and Alzheimer’s disease. 

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3. Identification of circadian rhythms in Nannochloropsis species using bioluminescence reporter lines

   https://doi.org/10.1111/tpj.14314  

   Poliner, Eric ; Cummings, Cameron ; Newton, Linsey ; Farré, Eva M. ( April 2019 , The Plant Journal)

   Circadian clocks allow organisms to predict environmental changes caused by the rotation of the Earth. Although circadian rhythms are widespread among different taxa, the core components of circadian oscillators are not conserved and differ between bacteria, plants, animals and fungi. Stramenopiles are a large group of organisms in which circadian rhythms have been only poorly characterized and no clock components have been identified. We have investigated cell division and molecular rhythms inNannochloropsisspecies. In the four strains tested, cell division occurred principally during the night period under diel conditions; however, these rhythms damped within 2–3 days after transfer to constant light. We developed firefly luciferase reporters for the long‐term monitoring ofin vivotranscriptional rhythms in twoNannochlropsisspecies,Nannochloropsis oceanicaCCMP1779 andNannochloropsis salinaCCMP537. The reporter lines express anticipatory behavior under light/dark cycles and free‐running bioluminescence rhythms with periods of ~21–31 h that damped within ~3–4 days under constant light. Using different entrainment regimes, we demonstrate that these rhythms are modulated by a circadian‐type oscillator. In addition, the phase of free‐running luminescence rhythms can be modulated pharmacologically using aCK1 ε/δ inhibitor, suggesting a role of this kinase in theNannochloropsisclock. Together with the molecular and genomic tools available forNannochloropsisspecies, these reporter lines represent an excellent system for future studies on the molecular mechanisms of stramenopile circadian oscillators.

    

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4. Circadian Synchrony: Sleep, Nutrition, and Physical Activity

   Healy, Kelly L. ; Morris, Andrew R. ; Liu, Andrew C. ( October 2021 , Frontiers in network physiology)

   The circadian clock in mammals regulates the sleep/wake cycle and many associated behavioral and physiological processes. The cellular clock mechanism involves a transcriptional negative feedback loop that gives rise to circadian rhythms in gene expression with an approximately 24-hour periodicity. To maintain system robustness, clocks throughout the body must be synchronized and their functions coordinated. In mammals, the master clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN is entrained to the light/dark cycle through photic signal transduction and subsequent induction of core clock gene expression. The SCN in turn relays the time-of-day information to clocks in peripheral tissues. While the SCN is highly responsive to photic cues, peripheral clocks are more sensitive to non-photic resetting cues such as nutrients, body temperature, and neuroendocrine hormones. For example, feeding/fasting and physical activity can entrain peripheral clocks through signaling pathways and subsequent regulation of core clock genes and proteins. As such, timing of food intake and physical activity matters. In an ideal world, the sleep/wake and feeding/fasting cycles are synchronized to the light/dark cycle. However, asynchronous environmental cues, such as those experienced by shift workers and frequent travelers, often lead to misalignment between the master and peripheral clocks. Emerging evidence suggests that the resulting circadian disruption is associated with various diseases and chronic conditions that further circadian desynchrony and accelerate disease progression. In this review, we discuss how sleep, nutrition, and physical activity synchronize circadian clocks and how chronomedicine may offer novel strategies for disease intervention. 

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5. Reconstitution of an intact clock reveals mechanisms of circadian timekeeping

   https://doi.org/10.1126/science.abd4453  

   Chavan, Archana G. ; Swan, Jeffrey A. ; Heisler, Joel ; Sancar, Cigdem ; Ernst, Dustin C. ; Fang, Mingxu ; Palacios, Joseph G. ; Spangler, Rebecca K. ; Bagshaw, Clive R. ; Tripathi, Sarvind ; et al ( October 2021 , Science)

   Circadian clocks control gene expression to provide an internal representation of local time. We report reconstitution of a complete cyanobacterial circadian clock in vitro, including the central oscillator, signal transduction pathways, downstream transcription factor, and promoter DNA. The entire system oscillates autonomously and remains phase coherent for many days with a fluorescence-based readout that enables real-time observation of each component simultaneously without user intervention. We identified the molecular basis for loss of cycling in an arrhythmic mutant and explored fundamental mechanisms of timekeeping in the cyanobacterial clock. We find that SasA, a circadian sensor histidine kinase associated with clock output, engages directly with KaiB on the KaiC hexamer to regulate period and amplitude of the central oscillator. SasA uses structural mimicry to cooperatively recruit the rare, fold-switched conformation of KaiB to the KaiC hexamer to form the nighttime repressive complex and enhance rhythmicity of the oscillator, particularly under limiting concentrations of KaiB. Thus, the expanded in vitro clock reveals previously unknown mechanisms by which the circadian system of cyanobacteria maintains the pace and rhythmicity under variable protein concentrations. 

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